This invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to an automatic transmission having a construction in which a semi-direct coupling clutch is interposed between a rotary member on the input side of a torque convertor and a rotary member on the output side and can mechanically transmit the torque between these rotary members while permitting the slip of rotation between the rotary members and the interconnecting force of this semi-direct coupling clutch is controlled in an interlocking arrangement with the torque convertor in such a manner that the degree of slip of rotation between both rotary members is adjusted in response to the transmission ratio of an auxiliary transmission effecting the speed change operation in accordance with one of transmission ratios of a plurality of stages. The automatic transmission of the present invention can reduce the fuel consumption of the an engine and ensures quiet and smooth driving of a vehicle.
If in a vehicular automatic transmission equipped with a torque convertor, a rotary member on the input side of the torque convertor such as a pump vane wheel and a rotary member on the output side such as a turbine vane wheel are coupled directly and mechanically by a clutch at the stage where the torque amplification by the torque convertor becomes unnecessary, the fuel consumption of the engine can be reduced and a quiet operation of the power transmission system can be obtained. It has been proposed to provide a certain degree of slip of rotation between the input side rotary member and the output side rotary member in order to secure the feeling of smooth driving during the direct coupling operation between both rotary members by a direct coupling mechanism. A direct coupling clutch which can permit such a certain degree of slip of rotation between both rotary members will be hereinafter referred to as a "semi-direct coupling clutch". An example of the semi-direct coupling clutch is disclosed, for example, in Japanese Patent Application No. 157263/1980 filed by the Applicant assignee of the present invention. The semi-direct coupling clutch of this prior art reference includes a recessed tapered surface rotating together with one of the input and output side rotary members, a convex tapered surface rotating together with the other of the rotary members and a plurality of rollers disposed inside a gap defined between the two tapered surfaces in the circumferential direction so as to be twisted with respect to the axes of rotation of both rotary members. A semi-direct coupling clutch of this type can permit the slip of rotation in a satisfactory manner against an excessive transmitted torque and can smoothly transmit the power. However, there are current requirements to reduce more effectively the fuel consumption and to accomplish quieter and smoother power transmission than previously obtainable.